Is there a way to overwrite a filter used in canonical.php?

Basically I created a mu-plugin in wp-content/mu-plugins. Here is a modified version of what I did to go to a custom file:

<?php
/*
    Plugin Name: Multisite - Sign-up Page
    Version: 1.0
    Author: Shawn
*/

/*
 * Redirect to correct domain signup (init runs after WordPress has finished loading but before any headers are sent)
 */
function multisite_signup(){
    if($_SERVER['REQUEST_URI'] == '/wp-signup.php'){
        do_action('signup_header', $_SERVER['HTTP_HOST'].'/my_custom_file.php');
    }
}
add_action('init', 'multisite_signup');

/*
 * In multisite this runs when the "main domain" signup.php is accessed OR a domain is not found
 */
function signup_redirect($url="#some_default#"){
        wp_redirect($url);
        exit;
}
add_action('signup_header', 'signup_redirect');